The conventional cipher technologies which are designed to ensure security of communication include a privacy key cipher system that makes use of a common privacy key which is known only to proper parties of communication, and a public key cipher system that has recourse to a pair of keys, given by a privacy and a public key.
Cryptology of the public key cipher system relies for its secrecy retainability on the fact that for example, factoring an extremely large integer is hard to compute. However, progresses in performance of computers, and advancements in distributed processing techniques using networks have made security by that system no longer perfect.
In contrast, if a privacy key known only to legitimate parties of communication can be held in common by the sender and recipient, an absolutely secure communication may be realized. In such recognition in the art, a technique called quantum cipher communication has in the recent decade been proposed that seeks secrecy retainability for a privacy key to be distributed in a quantum-mechanical principle (J. Cryptology, 5, 3-28 (1992), C. H. Bennett et al).
If the principle of quantum mechanics is applied that an action to observe or measure necessarily gives a disturbance to an object observed or measured, any attempt by a wiretapper to wiretap gives a change in a signal being transmitted.
It therefore follows that monitoring a signal to find a change in it can divulge the presence of a wiretapper. In specific terms, using a quantum cipher allows a privacy key with high secrecy retainability to be prepared and held in common by and between parties at two sites spaced distant. For effecting a quantum ciphering operation or encryption it is conventional to use a light as the carrier of a transmission signal. And it has been the common practice to detect a signal light by the photon counting method, a light detecting technique, that uses a detector which in response to one or more photons incident upon generates electric pulses at a certain given probability called quantum efficiency.
However, the quantum cipher communication systems so far proposed that rely on the photon counting method have failed to be practical because of the problems unresolved, both on its principle and technologically, which accrue from the method.
First, the problem on the principle is that the inability to check up on the quantum mechanical state of a signal light once it has been transmitted prevents detecting a wiretap if the wiretapper has taken a sophisticated measure such as a quantum nondestructive measurement. Stated otherwise, since a wiretapper is able to read information on the number of photons in a signal light without giving a change in that number of the photons (the effect of a measurement does appear as a change in phase), it follows that counting only the number of photons after transmission allows wiretapping to remain unnoticed.
Further, there exists the technological problem that a detector is unavailable having a high quantum efficiency enough to meet with lights of 1.3 μm or 1.5 μm in wave length for common use now in light communication systems. Not only does a loss in detection reduce the transfer rate for data, but on the principle it does not give a distinction from an attempt to wiretap.
It is accordingly an object of the present invention to provide a quantum cipher communication system that permits measuring a signal being transmitted in its quantum mechanical state. It is also an object of the present invention to provide a quantum cipher communication system that permits detecting the signal being transmitted at a substantially high quantum efficiency.